1. Field of the Invention
The present invention relates to a gas regulating device for use in calibration of a gas analyzer, calibration arrangements for use in calibration of a gas analyzer, a method for calibrating a gas analyzer, and use of a gas regulating device for calibration of a gas analyzer.
2. Description of the Prior Art
Gas analyzers are well known in the art and often used in various breathing assist devices such as ventilators and anesthetic machines to analyze the composition of breathing gases supplied to a patient and/or the composition of gases exhaled by the patient.
There are mainly two types of gas analyzers used for this purpose; in-line gas analyzers and side-stream gas analyzers.
An in-line gas analyzer, sometimes also referred to as a main stream gas analyzer, is positioned in the gas flow path of the breathing assist device, normally by connecting the gas analyzer in line with the gas-conducting tubing of the breathing assist device. Thereby, an in-line gas analyzer is automatically traversed by the gas that is to be analyzed.
A side-stream analyzer on the other hand is connected to the gas flow path of the breathing assist device through a side connection, for example a side connection in form of a thin hose connecting the side-stream gas analyzer with the Y-piece of the breathing assist device. Normally, a side-stream gas analyzer is hence not automatically traversed by the gas that is to be analyzed. Therefore, a side-stream gas analyzer typically comprises a pump or other flow generating means in order to “suck” the gas that is to be analyzed from the gas flow path of the breathing device to the measurement chamber of the gas analyzer where the gas is analyzed. Typically, a side-stream gas analyzer operates at a working flow in the range of 0 to 500 ml/minute, meaning that the flow generating means of the side-stream gas analyzer is adapted to generate a flow within that range.
In order to ensure reliable operation of gas analyzers they should be regularly maintained and calibrated. Calibration is typically performed once a year or so by connecting the gas analyzer to a calibration gas supply, typically in form of a gas cylinder comprising calibration gas having a very well-defined chemical composition. The gas analyzer is then calibrated such that the gas composition as determined by the gas analyzer corresponds to the well-defined composition of the calibration gas.
Known calibration arrangements used for calibrating side-stream gas analyzers suffer from disadvantages, some of which will be described below with reference to FIGS. 1 and 2.
FIG. 1 illustrates a calibration arrangement according to prior art for calibrating a side-stream gas analyzer 100. The side-stream gas analyzer is simply connected to a calibration gas supply in form of a gas cylinder 200, via a hose 300 and a T-piece 400. If the gas flow provided by the calibration gas supply 200 is below the working flow of the side-stream gas analyzer 100, the gas analyzer will start sucking ambient air through the open end 500 of the T-piece, which air will mix with the calibration gas and lead to inaccurate calibration of the gas analyzer. Of course, this is also what happens if the calibration gas cylinder 200 runs out of gas. If, on the other hand, the gas flow provided by the calibration gas supply 200 is too high, excess calibration gas will be discharged to the ambient environment through the free end 500 of the T-piece, which free end hence serves as an excess calibration gas outlet. This is an undesired scenario since a large volume of expensive calibration gas is wasted, which calibration gas, depending on the composition of the calibration gas, further may be inappropriate to release into the ambient environment. Some of these problems can be mitigated, at least to some extent, by connecting a flow meter 600 between the calibration gas supply 200 and the gas analyzer 100 so as to help a user of the calibration arrangement to adjust the supply of calibration gas to a suitable level. However, there will always be either a leakage of excess calibration gas, or an intake of ambient air, through the free end 500 of the T-piece.
To solve the problems of undesired mixing of ambient air and calibration gas, and discharge of calibration gas into the ambient environment, a calibration arrangement as shown in FIG. 2 has been suggested. In this arrangement, a gas collection bag 700 is slipped onto the free end 500 of the 1-piece. However, this is not a satisfactory solution since the calibration gas collected in the bag 700 can normally not in practice be returned to the calibration gas cylinder 200, and should not be returned since there will inevitably be a change in gas composition when disconnecting the bag from the T-piece. That the composition of the calibration gas collected in the bag is changed when disconnecting the bag from the 1-piece may also induce errors in the calibration of gas analyzers if the collection bag 700 is reused between different calibration procedures.
Another problem with this calibration arrangement may arise if the calibration gas cylinder 200 runs completely out of gas. When the calibration gas cylinder 200 is run empty of calibration gas, the gas analyzer 100 will start sucking calibration gas out of the collection bag 700 until the bag too is empty. Since the gas analyzer will continue to suck in gas even when the bag 700 is empty, a small negative pressure will occur in the hose 300 and 1-piece 400. This negative pressure may cause the calibration gas cylinder to discharge some more gas, thus creating a small negative pressure also within the gas cylinder. When disconnecting the gas analyzer 100 from the port of the 1-piece to which it is connected during calibration, the negative pressure in the hose 300, 1-piece 400 and calibration gas cylinder 200 will cause ambient air to be sucked in through this 1-piece port and leak into the calibration gas cylinder 200. If the empty calibration gas cylinder 200 is erroneously used in a subsequent calibration procedure, a small amount of the air that leaked into the gas cylinder 200 may be discharged and ruin the calibration result.